As a method for performing stable communication in terrestrial digital television broadcasting, a wireless LAN (Local Area Network), etc. via a transmission path in which delay dispersion caused by a multipath is involved, a multicarrier transmission system which is typified by an orthogonal frequency division multiplexing (OFDM) transmission system (hereinafter, referred to as an OFDM transmission system) has been widely used. On the other hand, in mobile communication in which a transmission apparatus, a reception apparatus, or both of the transmission apparatus and the reception apparatus perform communication while in motion, frequency fluctuations which are caused by a Doppler phenomenon and amplitude fluctuations of a received signal which result from delay dispersion caused by a multipath occur. In addition, in a frequency conversion section of the transmission apparatus or the reception apparatus, phase fluctuations may be caused by phase noise in an oscillator circuit. In the OFDM transmission system, modulated waves of subcarriers are densely multiplexed to be transmitted so that frequency spectra thereof are mutually overlapped. Therefore, in a case where the OFDM transmission system is used for the mobile communication, orthogonality among the subcarriers is impaired due to the above-mentioned frequency fluctuations, amplitude fluctuations, phase fluctuations, or the like, and inter-carrier interference among the subcarriers arises, leading to a problem of deterioration in communication quality.
As a conventional technique of alleviating the inter-carrier interference in the OFDM transmission system, a scheme described in a non-patent document 1 (hereinafter, referred to as a self-cancellation scheme) has been known. This self-cancellation scheme, by dividing a plurality of subcarriers in the OFDM transmission system into a plurality of groups, each of which has L subcarriers (L is an integer greater than or equal to 2) whose frequency allocation is continuous, allows a specific correlation in modulation of the L subcarriers in one and the same group. Thus, the inter-carrier interference is cancelled and suppressed.
Hereinafter, referring to figures, the self-cancellation scheme which is the conventional technique of alleviating the inter-carrier interference will be specifically described.
FIG. 11 is a block diagram illustrating a configuration of a transmission apparatus and a reception apparatus in an OFDM transmission system using the above-mentioned self-cancellation scheme. FIG. 12 is a diagram showing (a) an allocation state of modulation vectors which are arranged on a frequency axis and showing (b) the modulation vectors and allocation of signal points (on a complex plane) of demodulation vectors which have been canceling-demodulated in the OFDM transmission system using the self-cancellation scheme. In FIG. 11, a transmission apparatus 101 receives transmitted data, OFDM-modulates carriers based on the received transmitted data, and generates to output an OFDM signal. The OFDM signal outputted from the transmission apparatus 101 is received via a transmission path 103 by a reception apparatus 102. The reception apparatus 102 demodulates the OFDM signal received via the transmission path 103 and outputs received data.
The transmission apparatus 101 comprises a vector modulation section 111, a canceling modulation section 112, an IDFT (Inverse Discrete Fourier Transform) section 113, a guard interval addition section 114, and a frequency conversion section 115. A multicarrier modulation section 126 is constructed of the IDFT section 113 and the guard interval addition section 114. Hereinafter, operations of the sections of the transmission apparatus 101 will be described. In order to describe the gist of the conventional art in a concise manner, operations per symbol in the OFDM transmission system will be described.
Transmitted data of K bits is inputted to the transmission apparatus 101 per symbol in the OFDM transmission system. The transmitted data inputted to the transmission apparatus 101 is supplied to the vector modulation section 111.
The vector modulation section 111 receives the transmitted data of K bits. The vector modulation section 111 divides the inputted transmitted data of K bits into G groups, generates and outputs G modulation vectors based on the transmitted data of (K/G) bits which are given to each of the groups. Each of the modulation vectors outputted by the vector modulation section 111 contains the transmitted data of (K/G) bits. Here, K and G are integers greater than 0. If the self-cancellation scheme is described supposing that K is a multiple of G, there accrues no problem. Therefore, in the below description, K is supposed to be a multiple of G.
The G modulation vectors outputted by the vector modulation section 111 are supplied to the canceling modulation section 112. The canceling modulation section 112 allocates the G modulation vectors to the G subcarrier groups, respectively. Here, the G subcarrier groups are obtained by dividing N subcarriers into G groups, each of which has L subcarriers whose frequency allocation is continuous. An equation N=G×L is satisfied.
In the canceling modulation section 112, by using a polynomial P(D)=(1−D)(L-1) of a discrete filter wherein a delay element is D, coefficients of the filter are determined. In the above-mentioned polynomial of the discrete filter, an impulse response of the filter is represented. The coefficients of the filter can be obtained by expanding the polynomial P(D)=(1−D)(L-1). An expansion thereof is expressed as a polynomial with respect to D. When the order of the coefficients of D is P0, P1, P2, . . . P(L-1) in order from the order 0, the expansion is expressed as P(D)=P0+P1D+P2D2+ . . . P(L-1)D(L-1). The P0, P1, P2, . . . P(L-1) can be expressed by Pi (0≦i≦(L−1) and i is an integer).
As mentioned above, each of the G subcarrier groups has the L subcarriers. The L subcarriers have numbers from 0 to (L−1) in the order of frequencies with the smallest one first. The canceling modulation section 112 generates a modulation vector with a coefficient, which performs multicarrier modulation, in the i-th subcarrier ((0≦i≦(L−1) and i is an integer) among the subcarriers included in each of the subcarrier groups. The modulation vector with a coefficient can be obtained by multiplying the above-mentioned modulation vector, which is allocated to each of the groups, by the above-mentioned coefficient Pi. Thus, the canceling modulation section 112, based on the inputted G modulation vectors and the L coefficients Pi allocated to respective subcarriers, generates and outputs N modulation vectors with the coefficients (N=G×L).
In a case where L is 2, because P(D)=1−D is satisfied (i.e. P0=1, P1=−1), polarities of modulation vectors which each modulate two neighboring subcarriers are inversed (see FIG. 12(a)). Thus, a pair of modulation vectors whose polarities are inversed are provided. The pair of two modulation vectors are supposed to contain the same transmitted data. Also in the case where L is 2, G=N/2 groups, each of which has two neighboring subcarriers, are provided.
The IDFT section 113 included in the multicarrier modulation section 126 receives the N modulation vectors with the coefficients, which are outputted by the canceling modulation section 112. The IDFT section 113 subjects the received modulation vectors with the coefficients to inverse Fourier transform. The IDFT section 113 outputs, as a baseband OFDM signal, the signal obtained after the inverse Fourier transform.
The guard interval addition section 114 receives the baseband OFDM signal outputted by the IDFT section 113. The guard interval addition section 114 adds a guard interval signal to the received baseband OFDM signal and outputs the signal.
The frequency conversion section 115 receives the baseband OFDM signal to which the guard interval signal has been added. The frequency conversion section 115 frequency-converts the received baseband OFDM signal to a signal in a wireless frequency band and outputs the OFDM signal in the wireless frequency band. The OFDM signal outputted by the frequency conversion section 115 is supplied via an aerial wire to the transmission path 103 as the OFDM signal outputted by the transmission apparatus 101.
The above-mentioned OFDM signal which has gone through the transmission path 103 is supplied via the aerial wire to the reception apparatus 102.
The reception apparatus 102 includes a frequency conversion section 121, a guard interval removal section 122, DFT (Discrete Fourier Transform) section 123, a canceling demodulation section 124, and a vector demodulation section 125. A multicarrier demodulation section 127 is constructed of the guard interval removal section 122 and the DFT section 123. Hereinafter, operations of the sections of the reception apparatus 102 will be described. In order to describe the gist of the conventional art in a concise manner, operations per symbol in the OFDM transmission system will be described.
The OFDM signal which the reception apparatus 102 has received via the transmission path 103 is supplied to the frequency conversion section 121.
The frequency conversion section 121 receives the OFDM signal in the wireless frequency band, which the reception apparatus 102 has received. The frequency conversion section 121, by subjecting the received OFDM signal to down conversion, generates and outputs a baseband OFDM signal.
The guard interval removal section 122 receives the baseband OFDM signal outputted by the frequency conversion section 121. The guard interval removal section 122 removes a guard interval signal from the received baseband OFDM signal and outputs the resultant signal.
The DFT section 123 included in the multicarrier demodulation section 127 receives the baseband OFDM signal from which the guard interval signal has been removed. The DFT section 123, by subjecting the received baseband OFDM signal to Fourier transform, generates and outputs N demodulation vectors with coefficients.
The canceling demodulation section 124 receives the N demodulation vectors with coefficients. The canceling demodulation section 124 first divides the received N demodulation vectors with coefficients into G groups. Next, the canceling demodulation section 124 removes the coefficient Pi (0≦i≦(L−1) in each of the groups by multiplying the i-th demodulation vector with a coefficient by a reciprocal of the above-mentioned coefficient Pi, obtains a total sum from which the coefficient Pi has been removed, and generates to output a demodulation vector. As the demodulation vector, one vector for each of the subcarrier groups is generated and outputted. Therefore, G demodulation vectors for all of the subcarrier groups are generated and outputted.
The vector demodulation section 125 receives the G demodulation vectors outputted by the canceling demodulation section 124. The vector demodulation section 125 determines and outputs K pieces of demodulation data from the received G demodulation vectors.
The K pieces of demodulation data outputted by the vector demodulation section 125 is outputted as demodulation data from the reception apparatus 102.
Here, a principle of the technique of alleviating the inter-carrier interference by using the self-cancellation scheme will be described.
If frequency fluctuations occur on a subcarrier due to a Doppler phenomenon, inter-carrier interference between the subcarrier and a plurality of the other subcarriers is generated. An interference component of the inter-carrier interference has great correlation among subcarriers neighboring in a frequency axis direction. In other words, it has been known that when an interference coefficient of the inter-carrier interference which occurs between the i-th subcarrier and the k-th subcarrier due to the frequency fluctuations on the i-th subcarrier is S(i−k) and an interference coefficient of the inter-carrier interference which occurs between the (i+1)-th subcarrier and the k-th subcarrier due to the frequency fluctuations on the (i+1)-th subcarrier is S(i+1−k), there is a relationship S(i−k)≈S(i+1−k) between the interference coefficients (details are described in non-patent document 1).
As the simplest example, a case where the number L of subcarriers which the above-mentioned group has is 2 will be described. In the case of L=2, an expansion of the above-mentioned polynomial P(D) is P(D)=1−D, with P0=1 and P1=−1. Suppose that a modulation vector (provided from the vector modulation section 111) transmitted by two of the i-th subcarrier and the (i+1)th subcarrier which a subcarrier group has is X(i). Because of P0=1 and P1=−1, the canceling modulation section 112 performs canceling modulation by allocating P0X(i)=X(i) to the i-th subcarrier and P1X(i)=−X(i) to the (i+1) subcarrier. Here, a difference Sc between an amount of the inter-carrier interference occurring between the i-th subcarrier having the frequency fluctuations and the k-th subcarrier and an amount of the inter-carrier interference occurring between the (i+1)-th subcarrier having the frequency fluctuations and the k-th subcarrier is Sc=X(i)S(i−k)−X(i)S(i+1−k). When a right-hand side is arranged with a common term X(i), Sc=X(i)(S(i−k)−S(i+1−k)) results. There is the relationship S(i−k)≈S(i+1−k) between the interference coefficients of the inter-carrier interference S(i−k) and S(i+1−k) as mentioned above, and when a right-hand side is transposed to a left-hand side, S(i−k)−S(i+1−k)≈0 results. Therefore, the difference between the above-mentioned amounts of the inter-carrier interference is substantially zero (Sc≈0). In other words, the difference between the amount of the inter-carrier interference occurring between the i-th subcarrier having the frequency fluctuations and the k-th subcarrier and an amount of the inter-carrier interference occurring between the (i+1)-th subcarrier having the frequency fluctuations and the k-th subcarrier is substantially zero, and these amounts of inter-carrier interference are mutually canceled. Thus, generation of the inter-carrier interference is reduced.
Based on the above-described principle, the canceling demodulation section 124 included in the reception apparatus 102 cancels the inter-carrier interference.    [Non-patent document 1] Y. Zhao and S.-G. Haggman, “Intercarrier Interference Self-Cancellation. Scheme for OFDM Mobile Communication Systems”, IEEE Transactions on Communications, Vol. 49, No. 7, pp. 1185-1191, July 2001    [Non-patent document 2] J. G. Proakis, “DIGITAL COMMUNICATIONS second third edition”, pp. 548-557, McGraw-Hill    [Non-patent document 3] A. J. Viterbi, “Convolutional Codes and Their Performance in Communication Systems”, IEEE Transactions on communications, Vol. COM-19, pp. 751-772, 1971    [Non-patent document 4] L. R. Bahl, J. Cocke, F. Jelinek, J. Raviv, “Optimal decoding of linear codes for minimizing symbol error rate”, IEEE Transactions on Information Theory, Vol. 20, pp. 284-287, 1974